Hi Michael,
What exactly is the problem that you are having? How familiar are you with Eclipse and with adding plugins to Eclipse?
You just have to add the update site URL given at the homepage of MercurialEclipse (
https://bitbucket.org/mercurialeclipse/main/wiki/Home) to Eclipse update sites and select the plugin in the appropriate dropdown list. That's one the standard ways of adding plugins to an existing Eclipse installation. There's nothing specific to MercurialEclipse.
You may have to search the Web for a video or tutorial detailing the exact actions.
HTH.